The present invention relates to a heat-resistant coated optical fiber provided with a heat-resistant coating, which is employed in high-temperature/high-humidity environments.
As heat-resistant coated optical fibers usable in high-temperature environments, techniques disclosed in JP 1-173006A and JP 5-127052A have been known.
The former relates to a technique in which the outer periphery of an optical fiber is coated with a polyimide resin having a thickness of at least 25 xcexcm but not greater than 300 xcexcm, and states that its outer periphery is preferably further coated with a heat-resistant resin, such as a fluorine resin, equivalent to the polyimide resin. The latter is a technique in which the outer periphery of the optical fiber is coated with polyimide and polytitanocarbosilane. Both of them are described as capable of maintaining a high strength even under a high temperature by using polyimide, which is a heat-resistant resin, in their coating layers as such.
Examples of harsh environments where optical fibers are employed include high-temperature/high-humidity environments such as the inside of nuclear power plants, oil wells, and the like. It has been seen that, in the above-mentioned heat-resistant coated optical fibers, the heat-resistant coating resins and the optical fibers therein deteriorate in such high-temperature/high-humidity environments. This is because that, in a high-humidity environment with a humidity of 60%RH, for example, the imide group is hydrolyzed, whereby aromatic polyimide resins deteriorate. It has been seen that polyamic acid, which is a glass-decomposing gas generated upon this hydrolysis, deteriorates the optical fiber glass therein.
In order to overcome the above-mentioned problem, the heat-resistant coated optical fiber in accordance with the present invention comprises a first coating layer consisting of aromatic polyimide resin, covering an outer periphery of an optical fiber; a second coating layer consisting of silicone resin, covering an outer periphery of the first coating layer; and a third coating layer consisting of moisture-resistant resin, covering an outer periphery of the second coating layer and having a heat resistance equivalent to that of the first coating layer.
In accordance with the present invention, each of the first, second, and third coating layers has a heat resistance, so that the coating would not deteriorate even in high-temperature environments. Also, since the first coating layer, which may hydrolyze in a high-humidity environment, and the second coating layer are covered with the moisture-resistant third coating layer, steam is kept from penetrating into the first and second coating layers, whereby the deterioration in strength caused by hydrolysis can be suppressed. Further, the second coating layer can enhance the adherence between the first and third coating layers.
The thickness of the first coating layer is preferably at least 0.01 xcexcm in order to protect the surface of optical fiber glass against the moisture in atmosphere, and is preferably not greater than 20 xcexcm in order to improve productivity.
Also, the thickness of the second coating layer is preferably at least 5 xcexcm in order to suppress the hydrolysis and secure a favorable lateral pressure characteristic, and is preferably not greater than 200 xcexcm in order to suppress the deterioration in transmission characteristics caused by thermal shrinkage of the coating layer and the worsening of productivity.
Further, the thickness of the third coating layer is preferably at least 0.01 xcexcm in order to secure heat resistance/moisture resistance characteristics, and is preferably not greater than 500 xcexcm in order to suppress the deterioration in transmission characteristics caused by thermal shrinkage of the coating layer and the worsening of productivity.
The degree of cure of the first coating layer is preferably at least 0.30 in order to suppress the hydrolysis of polyimide.
From the viewpoint of heat resistance/moisture resistance characteristics and easiness in manufacture required for the third coating layer, it is preferably made of a fluorine resin.
A carbon coating may further be provided between the optical fiber and the first coating layer. In this configuration, glass-decomposing gases are kept from reaching the optical fiber even when the polyimide resin constituting the first coating layer is hydrolyzed.